Calling new URL-error message...Please Help!

programming flash
1

Hello There,
On the first frame of my timeline is the code for an MP3 player that exists in the 4 following frames, which are separated by frame labels. I am trying to call a different Mp3 URL in each different frame, but am getting this error msg: TypeError: Error #2007: Parameter url must be non-null.
Here is the code on the 1st frame:

var sndObject:Sound                = new Sound();
var chaObject:SoundChannel        = new SoundChannel() ;
var sndTransform:SoundTransform    = new SoundTransform();
var reqObject:URLRequest         = new URLRequest();
var sndBytes:ByteArray            = new ByteArray();
var intPosition:int             = 0;
var bolStop:Boolean                = true;

function initMP3Player(reqObject:String):void {
    // set event listeners
    //addEventListener(Event.ENTER_FRAME, renderSpectrum);
    
    var sndObject:Sound                = new Sound();
var chaObject:SoundChannel        = new SoundChannel() ;
var sndTransform:SoundTransform    = new SoundTransform();
//var reqObject:URLRequest         = new URLRequest();
var sndBytes:ByteArray            = new ByteArray();
var intPosition:int             = 0;
var bolStop:Boolean                = true;
    
    
    
    addEventListener(Event.ENTER_FRAME, calcProgress);
    this.mcMp3Player.btnStop.addEventListener(MouseEvent.CLICK, stopPlayback);
    this.mcMp3Player.btnPause.addEventListener(MouseEvent.CLICK, pausePlayback);
    this.mcMp3Player.btnPlay.addEventListener(MouseEvent.CLICK, startPlayback);
    this.mcMp3Player.btnMute.addEventListener(MouseEvent.CLICK, mutePlayback);
    this.mcMp3Player.btnUnmute.addEventListener(MouseEvent.CLICK, unmutePlayback);
    this.mcMp3Player.progress.addEventListener(MouseEvent.CLICK, setNewProgress);
    
    // use hand cursor for progress bar
    this.mcMp3Player.progress.buttonMode = true;
    this.mcMp3Player.btnPause.visible    = false;
    this.mcMp3Player.btnUnmute.visible    = false;
}

function stopPlayback(e:MouseEvent):void {
    chaObject.stop();
    sndObject = new Sound();
    bolStop = true;
    intPosition = 0;
    this.mcMp3Player.btnPlay.visible    = true;
    this.mcMp3Player.btnPause.visible    = false;
}

function pausePlayback(e:MouseEvent):void {
    this.mcMp3Player.btnPlay.visible    = true;
    this.mcMp3Player.btnPause.visible    = false;
    playSound(false);
}
function startPlayback(e:MouseEvent):void {
    this.mcMp3Player.btnPlay.visible    = false;
    this.mcMp3Player.btnPause.visible    = true;
    playSound();
}
function mutePlayback(e:MouseEvent):void {
    this.mcMp3Player.btnMute.visible    = false;
    this.mcMp3Player.btnUnmute.visible    = true;
    setVolume(0);
}
function unmutePlayback(e:MouseEvent):void {
    this.mcMp3Player.btnMute.visible    = true;
    this.mcMp3Player.btnUnmute.visible    = false;
    setVolume(1);
}

function setNewProgress(e:MouseEvent):void {
    var p:int = sndObject.length * e.currentTarget.mouseX / 220;
    chaObject.stop();
    chaObject = sndObject.play(p);
    this.mcMp3Player.btnPlay.visible    = false;
    this.mcMp3Player.btnPause.visible    = true;
}

function playSound(bolPlay:Boolean = true):void {
    if(bolPlay) {
        if(bolStop) {
            // load mp3 file if playback has been stopped
            sndObject.load(reqObject);
            bolStop = false;
        } else {
            intPosition = chaObject.position;
        }
        chaObject = sndObject.play(intPosition);
    } else {
        chaObject.stop();
    }
}

function setVolume(intVolume:int = 1):void {
    sndTransform.volume = intVolume;
    chaObject.soundTransform = sndTransform;
}

function calcProgress(e:Event):void {
    var p:MovieClip = this.mcMp3Player.progress.mcProgressFill;
    var w:int        = Math.round( 220 * chaObject.position / sndObject.length);
    p.width         = w;
}

And this is how I am calling it on 4 following frames:

initMP3Player("http://samueldixon.com/music/Let_Go.mp3");

Can anyone see what I am doing wrong I am really struggling to work it out!
Thanks in advance for any help!
Doodark x